Infrasound refers to sound vibrations that are at a frequency too low to be heard by the human ear which has a range from 20 Hz to 20,000 Hz (called the audible range). Levels below 20 Hz are described as infrasonic (infrasound) and those above 20,000 Hz are ultrasonic.
While we may experience discomfort at sounds we can hear at volumes of around 80 decibels upwards, it is believed exposure to low frequency sound vibrations which we cannot detect may also have considerable impact on humans. In much the same way many find the audible bass of a high volume car stereo annoying, sounds at even lower frequencies may interfere with our emotions and perceptions. It is known that military forces have examined the effects of infrasound and even looked into its use as a weapon.
Exposure to infrasound has been demonstrated to effect recipients with symptoms including fear, sorrow, depression, anxiety, nausea, chest pressure and hallucination. It can cause objects to move through vibration and some believe the body’s internal organs can be effected. It is suggested that levels above 80 decibels at frequencies between 0.5 to 10Hz may start to effect the vestibular of the inner ear thus causing disorientation. Any high volume sound can trigger the body to react by increasing respiration, heart rate and blood pressure, but when they cannot actually hear the sound recipients are left with no explanation for the sudden onset of these symptoms. This may then lead to further effects caused by the minds possible reaction to the unknown, as outlined below.
Once the mind receives information it considers unusual it may enter into “search mode” to try and explain what is being experienced, calling on all senses to assist - sight, sound, touch, smell etc... The longer the search goes on without an answer, the more intense the scrutiny. In the extreme, the body may react in “survival instinct” - fear sets in, pulse races etc... This is the body’s natural reaction to the unknown, preparing it for possible “fight or flight” from danger. At such times, because the senses are so heightened and “in tune” for experiencing something, the brain may begin to misinterpret what the senses are picking up. Much akin to sitting alone in the dark after watching a horror movie although to a much greater extent.
This is all a natural reaction of the brain and very real to the witness. Possible triggers (either alone or in combination) are anything that may suggest something strange is occurring including high EMF, infrasound, low atmospheric pressure, carbon monoxide exposure, darkness, isolation and any stimulus that may create suggestion such as watching a spooky movie, being in an spooky location, or Ouija board use. Ocean waves are known to sometimes generate infrasound and it has been suggested to have been a possible "trigger" causing ships crews to abandon their craft in fear, only to have the ship later found mysteriously drifting about unmanned.
The range of infrasound is generally accepted to be between 0-20 hertz with a specific area of interest between 17 and 19 hertz. Tests by NASA have revealed that the human eyeball resonates at around 18Hz, to which infrasound exposure may cause a reaction and lead to hallucinations.
Infrasound occurs quite naturally at some locations and possible causes include storms, earthquakes, waterfalls, volcanoes, ocean waves and wind reacting with structures such as chimneys. Some buildings or natural features can act as Helmholtz resonators and create infrasound at high levels. Ancient places of worship or ceremonial burial such as the Maeshowe mound in Orkney, have been shown to act in this way. Some animals are sensitive to these low frequency vibrations and may appear to "foresee" approaching storms and earthquakes. Elephants are known to use infrasound as a form of communication over long distances.
It is possible that any room with an open doorway or window can operate like a Helmhotz resonator, similar to blowing a column of air across an empty bottle. Subsonic sound can travel long distances, pass through walls and may be amplified in tunnel like structures. Standard hearing protection is of little use for subsonic sound as it often can pass straight through and may even be amplified. There have been links reported between supposedly haunted locations and the presence of infrasound, which is the reason paranormal investigators may monitor infrasound levels whenever possible.
The following text gives some insight into how sound levels including infrasound are represented (usually in pascals, micropascals or decibels) which may be of some assistance in interpreting the results of monitoring.
There is a huge variance in sound pressure ranging from the minimum that can be heard by the human ear, 20 μPa (micropascals), to the threshold of pain, 20 Pa (pascals). Because of this huge range a logarithmic scale is used to represent the sound pressure level (SPL). A reference of 20 μPa (micropascals) is commonly used, being the lowest level that can be heard by the human ear at a frequency of 1000 Hz. This is equal to .02 mPa (millipascals) or 0.00002 Pa (pascals). The unknown level is compared to the 20 μPa threshold which is given a value of 0 dB (decibels) and the resulting level is expressed in decibels (dB). Because the human ear perceives sound intensity differently depending on it’s frequency, weightings may also be applied in attempt to match what the human ear experiences. "A-weighted” levels are the most common used, although a “G-weighting” is perhaps more suitable for infrasound.
For comparison dB levels for some audible sounds are given below.
0-10dB Threshold of human hearing.
10-20dB Normal breathing, rustling leaves.
20-30dB Whispering at about 1.5 metres.
40-50dB Coffee maker, library, quiet office, quiet residential area.
50-60dB Dishwasher, electric shaver, office, rainfall, refrigerator, sewing machine.
60-70dB Air conditioner, alarm clock, background music, normal conversation, television.
70-80dB Coffee grinder, toilet flush, freeway traffic, hair dryer, vacuum cleaner.
80-90dB Blender, doorbell, heavy traffic, hand saw, lawn mower, ringing telephone, whistling kettle.
85dB Lower limit recommended for the wearing of hearing protection.
90-100dB Electric drill, shouted conversation, tractor, truck.
100-110dB Baby crying, boom box, factory machinery, motorcycle, subway train.
110-120dB Ambulance siren, car horn, leaf blower, walkman on high, power saw, shouting in the ear.
120-130dB Auto stereo, rock concert, chain saw, pneumatic drills, stock car races, thunder, power drill.
130-140dB Threshold of pain, air raid siren, jet airplane taking off, jackhammer.
150-160dB Artillery fire at 500 feet, balloon pop, cap gun.
160-170dB Fireworks, handgun, rifle.
170 -180dB Shotgun.
180 - 190dB Rocket launch, volcanic eruption.
